The present invention relates to apparatus for drying webs or sheets of paper, metallic or plastic foil, cardboard, woven or non-woven textile materials and the like. More particularly, the invention relates to improvements in apparatus for drying such webs or sheets with ultraviolet radiation.
Drying apparatus are normally utilized in conjunction with printing machines, such as screen printing machines, to expel moisture from the material which issues from the machine. These drying apparatus normally employ a large number of discrete ultraviolet lamps because the conicity of the beam of radiation issuing from an ultraviolet lamp is relatively small. The ulitization of a large number of lamps creates problems, particularly since the purpose of the lamps is often twofold, i.e., they must dry the material which issues (either continuously or intermittently) from a printing or other treating machine, and they must also ensure adequate polymerization of the applied substance or substances (such as one or more coloring agents). Moreover, it is often necessary to subject the materials to a relatively long-lasting drying action (this depends on the nature and quantity of substances which are applied to the material in a screen printing or other treating machine). While the material which issues from a screen printing machine could advance continuously, it is customary to advance such material stepwise, especially if the machine employs flat stencils or screens. This renders it necessary to deactivate the ultraviolet lamps during each interval of dwell of the treated material in order to prevent overheating and the resulting charring, burning, shrinkage and other undesirable consequences of the drying operation.
The likelihood of overheating the material in a conventional drying apparatus which employs ultraviolet lamps is especially pronounced because the generation of ultraviolet radiation is invariably accompanied by the generation of infrared rays which produce substantial amounts of heat. Therefore, many types of materials cannot be treated in heretofore known drying apparatus which utilize sources of ultraviolet radiation.
Attempts to avoid the deleterious effects of infrared rays issuing from sources of ultraviolet radiation include the provision of mirrors which are supposed to reflect ultraviolet light and the utilization of blowers which direct streams of a suitable coolant across the path of deflected radiation so as to remove a certain percentage of heat. A drawback of such proposal is that the efficiency of the drying apparatus is reduced considerably and that the energy consumption of the apparatus (in comparison to its output) is excessive. Moreover, the utilization of blowers adjacent to the path of movement of the web or sheet of material issuing from a screen printing or like machine brings about the drawback that the layer or layers of coloring matter on the material rapidly develop a skin which interferes with predictable drying and, at the very least, prolongs the drying operation. The skin constitutes an envelope which prevents the expulsion of moisture from the layer or layers therebelow.
A further drawback of presently known drying apparatus which employ a large number of ultraviolet lamps or burners is that the cost of such apparatus is prohibitive. The reason is that the initial cost of each lamp is considerable as well as that the useful life of an ultraviolet lamp is relatively short. The short useful life of ultraviolet lamps is attributable, to a certain degree, to the fact that the neighboring lamps adversely influence each other.
An additional drawback of presently known drying apparatus which employ large numbers of ultraviolet lamps is that it is difficult or impossible to properly array and orient the lamps so as to ensure uniform treatment of each and every portion of that length of the material which is exposed to ultraviolet radiation. The reason is that the beams of radiation issuing from neighboring lamps partially overlap each other so that certain portions of the material to be dried are invariably exposed to radiation of much greater intensity than the remaining portions of the material. Not only the material of an intermittently or continuously running web or sheet of paper, textile material or the like reacts differently to exposure to radiation of different intensity but this applies with equal force to the behavior of coloring media which are applied to such material in a screen printing or like machine.
Intensive heating of certain portions of or the entire material that issues from a screen printing or other treating machine is likely to damage the material in a number of not readily detectable ways. For example, overheating can entail a weakening of the affected portions of the material in a manner which is not immediately detectable. Thus, the overheated portions of the material are likely to become brittle so that their useful life is much shorter than that of the remaining portions of the same material.
Still another drawback of presently known drying apparatus which employ batteries of ultraviolet lamps is that their output is relatively low. This is due primarily to the need for frequent replacement of one or more lamps. As stated above, the useful life of an ultraviolet lamp is relatively short and, if the apparatus employs a large number of such lamps, it is likely to be idle for extended intervals of time or, more accurately stated, the apparatus must be arrested again and again whereby the total number of down times adds up to a substantial part of a shift. The failure of a single lamp invariably necessitates a stoppage of the apparatus and replacement of the defective lamp.
British Pat. No. 386,840 discloses a drying apparatus which is shiftable transversely of the path of the material to be dried. Such shiftability is proposed for the purpose of ensuring that the lamps will be moved to optimum positions relative to the material of the web or sheet. The patent proposes the utilization of guide rails to facilitate the transport of drying apparatus to a position wherein radiation issuing from the lamps is intercepted by a plate so that an operator can extend her or his hand into the apparatus in order to ascertain the progress of the drying operation. Once the apparatus is in actual use, the lamps or burners are at a standstill and direct radiation in a particular direction which cannot be changed. This brings about the same drawbacks as those of the aforedescribed stationary drying machines.